1. Field of the Invention
The invention generally relates to a package for an electronic device, a base substrate, an electronic device, and a fabrication method thereof.
2. Description of the Related Art
Recently, there has been a demand to downsize electronic elements mounted to electronic devices and improve the performance thereof with downsizing and high performance of the electronic devices. For instance, there have been similar demands on surface acoustic wave (SAW) devices that are electronic parts used as filters, delay lines, oscillators in electronic devices capable of transmitting and receiving radio waves.
Generally, SAW devices have a SAW filter chip that is flip-chip mounted within a cavity having a bottom surface on which interconnection lines are formed. An interdigital transducer (IDT) having comb-like electrodes is formed on a piezoelectric substrate of the SAW filter chip. This type of SAW devices is disclosed in Japanese Laid-Open Patent Application Publication 2001-53577, particularly, FIG. 3. In the SAW device of the above-mentioned type, an input electric signal applied to the IDT located on the input side of the device is transformed to a SAW, which is then propagated on the piezoelectric substrate. A filtered electric signal is available via the IDT located on the output side.
A cavity that houses the SAW filter chip is required to be hermetically sealed in order to reduce loss of the SAW that is propagated on the piezoelectric substrate. There is a method for sealing the cavity shown in FIG. 1. A washer 116 of solder made of gold-tin alloy is provided on sidewalls that define cavity. The washer 116 serves as a bonding material. The washer 116 is then heated and pressurized, so that a lid or cap 115 placed on the washer 116 can be fixed to the sidewalls.
The lid 115 may be made of an electrically conductive material in order to reduce external electromagnetic noise. In that case, it is required that the lid 115 is set at the ground potential to prevent degradation of performance of the filter. This requirement may be achieved in the conventional method shown in FIG. 1. More particularly, castellations 108 are provided at the four corners of the package 102 so as to reach the backsurface of the package 102. A conductor is provided on the castellations 108, via which the lid 115 coupled to a pattern 109 on the top of the package 102 can be electrically connected to a ground pattern formed on a circuit board provided at the bottom side of the package 102.
However, the above conventional method has the following disadvantage. That is, melted washer 116 flows out through the castellations 108, and may short-circuit a wiring pattern (foot pattern) formed on the backside of the package 102 or a wiring pattern formed on a die attach surface that corresponds to the bottom surface 106 of the cavity 105. Thus, the step of fixing the lid 115 is extremely sensitive.